Question

why distance has to be more then size of the charge for charge extended bodies for application of coulombs force

Answer 1

For finding the electrical force between two charged bodies we assume that the distance between them is much larger than their sizes. If the distance is small or comparable, then the charge distribution on one body will change because of the presence of other charged body, be both same or opposite nature of charges.
In those cases we would need to use Gauss's Law to find the Electric field or force.